This invention relates to a disc centering device for centering a disc in order to accurately mount the same on a turntable in a disc playback device.
As is widely known, in a playback device for a disc such as a video disc, a digital audio disc, etc., it is necessary to align the center of a disc with the rotational center of a turntable or with the center of a disc motor shaft when the disc is mounted on the turntable in order to precisely read out recorded signals by a pickup head. There have been proposed devices shown in FIGS. 5 and 6, for example, as the disc playback system with the above mentioned purpose.
In the disc playback device shown in FIG. 5 (proposed by the U.S. Pat. No. 4,340,955), the reference numeral 15 denotes a chassis. A drive motor 16 is fixed on the lower surface of the chassis 15. A shaft 17 of the drive motor 16 projects upwardly from the upper surface of the chassis 15, and a turntable 18 is fixedly mounted on the shaft 17. The turntable 18 comprises a disc mounting member 20 which is provided on a main body 19 at the upper edge thereof, the main body 19 being in the form of a cylinder with a bottom. The shaft 17 is attached with a centering member 22 having a tapered surface 21 on the outer circumference thereof. The centering member 22 is bored with a hole 23 through which the member 22 is mounted on the shaft 17, is guided by the shaft 17 to be freely movable in the vertical direction within a recess 24 of the turntable 18, and is constantly urged upwardly with a spring 25 once the upper limit is determined. In the figure, the reference numberal 26 denotes a disc, and 27 a clamper which clamps the disc 26 on the turntable 18.
In order to mount a disc 26 on the above mentioned device, the central hole of a disc 26 is fitted on the tapered surface 21 of the centering member 22, and the disc 26 is pushed down with the clamper 27 together with the centering member 22, and then is held between the lower portion of the clamper 27 and the disc mounting member 20 of the turntable 18. This type of device can accurately center a disc placed thereon, if the central hole of the disc 26 is uniformly larger or smaller than a reference diameter as the tapered surface 21 of the member 22 is constantly engaged with the central hole of the disc.
In the disc playback device shown in FIG. 6 (proposed by the assignee of this application in the U.S. patent application Ser. No. 665,931), the reference numberal 28 denotes a motor shaft, and 29 a turntable fixed on the motor shaft 29. The turntable 29 is a disc-like member and is formed with an annular center spindle 32 on the main portion 30 thereof. The center spindle 32 defines a tapered surface 31 on the upper surface thereof and is provided with a disc mounting member 34 on the outer side thereof for carrying a disc 33 thereon. The main portion 30 is formed with plural recessed portions 35 (only one of them is shown in the figure) at uniform intervals in the circumference. The recessed portions 35 are so formed as to extend in radial direction and to separate the center spindle 32 at the inner ends thereof. Supporting arms 37 are cantilevered to extend radially inwardly of the main portion 30 within the recessed portions 35. The supporting arms 37 have engagement projections 38 on their ends near the center spindle 32 to protrude upwardly. The engagement projections 38 are tapered upwardly on the upper outer circumference thereof to form a tapered surface 39. The turntable 29 is an integrally molded member made of synthetic resin. The supporting arms 37 can be deflected on the tip ends thereof in the vertical direction as shown with an arrow in the figure. The reference numeral 40 denotes the disc stabilizer which fixes the disc 33 on the turntable 29.
In mounting the disc 33 on the above device, the central hole of the disc 33 is guided by the tapered surface 31 to be fitted on the center spindle 32, and is simultaneously engaged with the tapered surface 39 of the engagement projection 38 formed on the supporting arm 37. Then, the disc stabilizer 40 is pressed onto the upper surface of the disc 33 so as to hold the disc 33 between the stabilizer 40 and the turntable 29. In this type of device, the ends of the supporting arms 37 are deflected downwardly depending upon the diameter of the central hole of the disc 33 so as to cause the tapered surface 39 of the engagement projection 38 to become engaged with the central hole of the disc 33 on the periphery thereof, thereby precisely centering the disc 33.
The disc playback device shown in FIG. 5 is defective, however, in that the turntable 18 and the centering member 22 call for high precision manufacture and therefore push up the cost because these members are fixed and mounted on a common shaft 17 in a movable manner relative to each other. Further, as the disc is centered in this type of device on the surface of the centering member 22 which is uniformly tapered on all the circumference thereof, if irregular fins are formed within the center hole of the disc, precise centering becomes impossible.
The disc playback device shown in FIG. 6 is defective, moreover, in that as a disc is centered by utilizing the flexibility of the supporting arms 37, deformation occurring on the supporting arms 37 due to chronological changes or heat may deteriorate the centering capability. The system is further detrimental in that the radical displacement is limited on the location where the tapered surface 39 abuts onto the disc 33 to thereby offer only a limited centering capability.
In the devices shown in FIGS. 5 and 6, since the portion which mainly centers a disc also acts as the member urging the disc in the upward direction, the disc is applied with a force which works to lift the disc away from the turntable to substantially reduce the clamping force on the disc.